FIG. 1 diagrammatically depicts a conventional installation for a measurement system 300. The measurement system 300 includes a probe or sensing element 301. The sensing element 301 is mounted on, and extends into a tank 302 that holds a flammable or combustible fluid such as gasoline. The measurement system 300 can be used to determine the level of material in the tank 302.
The measurement system 300 also includes a housing or junction box 304. The junction box 304 encloses a junction between electrical leads from the sensing element 301 and an electrical cable 308. The cable 308 can carry electrical signals generated by the sensing element 301 to signal processing equipment 310 located in a control room 312 or other location remote from the tank 302. The cable 308 is housed in a protective metal conduit 314 that extends between the junction box 304 and the control room 312.
The control room 312 is designated a “non-hazardous” area, in accordance with local guidelines such as the National Electrical Code (NEC).
The sensing element 301 is normally exposed to the flammable or combustible fluid within the tank 302, and is therefore considered to reside in an area designated “hazardous” in accordance with local guidelines such as the NEC.
The sensing element 301 is equipped with a seal 320 that, under normal operating conditions, prevents the fluid in the tank 302 from leaking past the sensing element 301 and into the junction box 304. Leakage in the seal 320, however, can permit the fluid to migrate into the junction box 304 and the attached conduit 314. The sensing element 301, which is directly exposed to the fluid within the tank 302 during normal operating conditions, is configured to be absent of any sources of energy that can potentially ignite the fluid. This type of configuration is characterized as “Intrinsically safe,” in accordance with local guidelines such as the NEC.
The junction box 304, the conduit 314, and the seal fittings 319 (poured seals containing porous material, e.g., CHICO®) are components within the “explosion proof” system, in accordance with local guidelines, such as the NEC. Moreover, the conduit 314 and the junction box 304 are made sufficiently robust to withstand the relatively high internal pressure that may occur during an explosion.
Although the seal fittings 319 can restrict energy from an explosion, the porosity of the seal fittings 319 permits fluid to pass therethrough. Thus, fluid that leaks through the seal 320 of the sensing element 301 can potentially reach the control room 312 by way of the junction box 304 and the conduit 314.
A second, or secondary seal 330 can be mounted on the conduit 314, to prevent the leaked fluid from migrating to the control room 312 by way of the conduit 314. The secondary seal 330 can include a housing through which the cable 308 is routed. The interior of the housing can be potted once the cable 308 has been routed therethrough.
The secondary seal 330 is located in a hazardous area and is a component in a system employing explosion proof as the method of protection. Therefore, the seal 330 must be able to withstand relatively high internal pressure in order to contain an explosion that may occur. The secondary seal 330 is thus larger, more robust, and more expensive than a seal intended for use in an area not designated as using explosion proof as the method of protection. Moreover, the potting of the secondary seal 330 is normally performed in the field as the secondary seal 330 is being installed, thereby making the time and effort needed to install the seal 330 greater than would otherwise be required.
The secondary seal 330 can potentially be exposed to an overpressure condition if substantial leakage occurs through the seal 320, particularly when the tank 302 is pressurized. Excessive pressure can damage the secondary seal 330, which in turn can result in migration of the leaked fluid past the secondary seal 330 and into the control room 312.
Pressure relief for the secondary seal 330 can be provided by configuring the junction box 304 to crack or fracture at a predetermined pressure that is lower than the maximum rated operating pressure of the secondary seal 330. Because the junction box 304 is a component in an explosion proof system, the pressure at which the junction box 304 will crack or fracture needs to be higher than the internal pressure that the junction box 304 must contain during an explosion, thereby increasing the maximum rated pressure of the secondary seal 330 to higher level than would otherwise be required.